The present invention relates in general to a variable-length encoding-decoding system and, in more particular, to a variable-length encoding-decoding system applied to enhance the efficiency of recording/reproduction of digital information on a recording medium.
When binary digital data stream such as digital audio information is recorded on a predetermined recording medium such as a magnetic tape, binary data is modulated or encoded for recording so as to increase a recording density of the recording medium. The 3 position modulation (called 3PM) technique is known as one of the encoding techniques. According to this modulation technique, binary data stream is framed in units of three bits. The 3-bit data word is converted into a 6-bit encoded word (code word). A recording signal is subjected to transition so as to correspond to binary data of logic "1" among logic levels "1" and 37 0" contained in the encoded word. The recording signal obtained in accordance with the 3PM technique has a pulse waveform which changes between two predetermined levels. According to this modulation technique, at least two "0"s are present between adjacent logic "1"s in a succession of code words. When the logic "1" data are subjected to transition and are recorded on the recording medium, the number of times of transition can be decreased as compared with the case where the original binary data stream is recorded thereon. This fact contributes to an increase in data recording density.
When transition is performed in synchronism with the logic "1" data included in the succession of code words, it is required that a minimum distance of the transitions (i.e., minimum transition period) (Tmin) of signal polarities is longest and, at the same time, a maximum distance of transitions (i.e., maximum transition period) (Tmax) is shortest. Under the condition that the recording signal is subjected to transition in accordance with the succession of code words, the longer the minimum transition period (Tmin) (i.e., the minimum time for two successive transitions of the signal levels to occur) becomes, the higher the data recording density of the recording medium becomes. However, in the case where the maximum transition period is long, it is difficult to extract the clock signals in the reproduction mode of the recorded signal. In addition to this drawback, the necessary configuration of the system becomes complex. In other words, it is easy to extract the clock signal from the recorded signal when the maximum transition period Tmax is shortest. From this point of view, when the bit cell period of the data word is given to be T, the minimum transition period Tmin becomes 1.5 T and the maximum transition period Tmax becomes 6T in the 3PM technique. Therefore, when the data stream is encoded by the above-mentioned modulating method to record the encoded data on the recording medium, extraction of the clock signal cannot be easily carried out, resulting in inconvenience.
The above problem is overcome by a variable-length encoding-decoding technique disclosed in U.S. Pat. No. 3,689,899. According to this technique, the data stream is encoded such that a ratio of the number of data words to the number of code words is fixed between 1:2 while the block length of the data word varies between 2 bits, 3 bits and 4 bits. According to the teaching of this U.S. Patent, the maximum transition period Tmax can be improved to be up to 4T. However, this encoding-decoding system requires an associative memory and becomes bulky. The circuit arrangement of this system becomes complex, which entails another problem. Although the encoding-decoding efficiency can be improved, the circuit configuration of the system becomes complex. As a result, it is not expected that this system will be applicable to a digital audio recording/reproduction system such as a digital audio tape recorder which uses a magnetic tape that has been recently developed.
In addition to the technique disclosed in the above-mentioned U.S. Patent, the encoding-decoding techniques which resemble the 3PM technique are disclosed in Japanese Patent Disclosure (KOKAI) Nos. 54-80726, 56-101614, 57-69959 and so on. These conventional encoding-decoding systems cannot simultaneously satisfy the requirements of encoding-decoding efficiency and a simple circuit arrangement.